It’s been 50 years since Tom Kielbania was onstage at the Catacombs, a cramped, wood-paneled jazz club in the basement of a building on Boylston Street, performing for an audience of perhaps 40 people.

Kielbania, a student at Berklee College of Music, played upright bass that night in a trio that included John Payne, a Harvard dropout, on flute, and the enigmatic Irish singer Van Morrison strumming an acoustic guitar.

Morrison, just 22 at the time, debuted songs that would be released a few months later on “Astral Weeks,” a mesmerizing album that’s achieved legendary status in the pantheon of pop music. Incredibly, the show was recorded, captured on reel-to-reel tape by Morrison’s friend, future J. Geils Band singer Peter Wolf, but no one, including Kielbania and Payne, had ever heard it.

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Until Ryan Walsh played it for them, that is. Walsh is the author of “Astral Weeks: A Secret History of 1968,” an exhaustively — some might say obsessively — researched account of the album’s origin story, beginning with Morrison’s arrival in Cambridge in early 1968 and the mercurial singer’s abrupt departure several months later.

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But the book, due out Tuesday, is about much more than that. It places the creation of Morrison’s masterpiece in the context of what else was happening in Boston, name-checking everyone from Thomas Hart Benton and Timothy Leary to Jonathan Richman and James Brown. Of all the books that will be published on the 50th anniversary of 1968, it’s safe to say Walsh’s is the only one with a story to tell involving both the Velvet Underground, who were a fixture at the Boston Tea Party that year, and Tony Curtis, the unlikely star of the 1968 film “The Boston Strangler.”

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“Ryan did a lot of digging to put this book together,” says Kielbania, 70. “He’s done a fantastic good deed.”

Walsh, who grew up in Dedham and graduated from Boston University, is not a journalist. By day, he’s the marketing manager for ArtsEmerson, and by night the frontman for Hallelujah the Hills, an indie rock band that takes its name from the 1963 cult comedy directed by Adolfas Mekas. Walsh is also married to singer Marissa Nadler, who, not insignificantly, put “Astral Weeks” on the turntable at the end of their first date in 2010.

“I already liked her, but I thought, ‘Wow, this is a good sign,’” says Walsh, sitting at the kitchen table in the couple’s apartment in Jamaica Plain.

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Walsh, who’s 39, has always felt an intense connection to “Astral Weeks,” and he’s not alone. Critics agree that the LP, with its dreamy acoustic arrangements and seemingly stream-of-consciousness lyrics, ranks among the greatest ever. Singer Elvis Costello has called it “the most adventurous record made in the rock medium.” But the backstory — why Morrison and his girlfriend Janet Planet moved to Massachusetts and what the singer did while he was here — had never been adequately explained.

Keith Bedford/Globe Staff

Ryan Walsh, who has written the book “Astral Weeks: A Secret History of 1968,” at his home in Jamaica Plain.

“The album meant so much to me emotionally. I felt indebted to it in a way,” says Walsh. “I’d read snippets online about Van being in Boston before making ‘Astral Weeks,’ but biographies only devote a page or two to it.”

In 2015, Boston Magazine devoted six pages to it, publishing a marvelously reported piece by Walsh detailing Morrison’s escape from New York in 1968 — his liaison at the record label had ties to the mob — and the genesis of the singer’s new, unplugged sound: While living on Green Street near Central Square, Morrison claimed he had a dream that there were no more electric instruments. Walsh’s entertaining tale was widely shared on social media, where it caught the eye of Ed Park, a serious “Astral Weeks” fan who was then the executive editor of Penguin Press.

“The way Ryan told the story, the way he’d reported it out, yielded so many great characters,” says Park. “You could tell he was writing this story because it meant something to him. I didn’t know if there was a book there, but I wanted to talk to him.”

It was decided that a whole book about Morrison’s brief tenure in Boston would be, in Walsh’s words, “thin soup,” but perhaps he could craft a narrative with multiple, intersecting stories about other forgotten people and places that composed the city’s flourishing counterculture at the time. He started with Mel Lyman, the charismatic leader of the Fort Hill Community, a commune of sorts whose members still reside in a cluster of buildings near the Gothic-style tower in Roxbury known as the Cochituate Standpipe.

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“I was looking for stories and people that interested me,” Walsh says. “Van and Mel were living a mile from each other during the same year and both were pursuing spirituality through music.”

‘The album meant so much to me emotionally. . . . Writing this book was the hardest thing I’ve ever done, but also the most rewarding.’

There’s no evidence Morrison and Lyman, who played banjo and harmonica in the Jim Kweskin Jug Band, ever met, let alone knew each other, but that doesn’t matter. Walsh is fascinated by the Fort Hill Community, its underground newspaper Avatar, and its early adherents, including characters like Mark Frechette, a self-described “hippie dropout bum” who was living on Fort Hill when Italian director Michelangelo Antonioni inexplicably cast him as the lead in the movie “Zabriskie Point.” (Frechette was later arrested during a botched bank robbery in Roxbury and died in prison.)

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Copies of Avatar, an underground Boston newspaper.

And there’s more: Walsh weaves in the story of the Bosstown Sound, the trademarked label given to Ultimate Spinach and other Boston-based psychedelic bands by producer Alan Lorber; chronicles the making of “Titicut Follies,” director Frederick Wiseman’s documentary exposing the grim conditions at the Bridgewater State Hospital for the criminally insane; and recollects “What’s Happening, Mr. Silver?,” a WGBH talk show that survived for 34 bewildering episodes.

“Not to sound obsequious, but I was singularly impressed with Ryan,” says David Silver, the show’s madcap host who lives now in upstate New York. “It can be difficult to translate idiosyncratic art, but Ryan is thoroughly familiar with the zeitgeist of the time.”

In all, Walsh interviewed more than 100 people, including Jonathan Richman, who was a Natick teenager in 1968 and very much aware of Morrison. Richman, who went on to form the Modern Lovers, rarely grants interviews, but he communicated with Walsh through the mail, responding to the author’s questions with letters that had musings scribbled on the envelope.

Walsh was the last person to interview “Astral Weeks” producer Lewis Merenstein, who died in 2016, and he managed to speak to every musician known to have played with Morrison during his time in Boston.

In the book, Walsh also recounts a memorable evening spent swilling bourbon and playing records with Peter Wolf, but he says Wolf is not the person who slipped him the digitized copy of the 1968 Catacombs performance, a set that includes stirring versions of two “Astral Weeks” songs, “Cyprus Avenue” and “Madame George.” Walsh won’t reveal who gave him the recording.

His attempts to speak with Morrison himself — Walsh contacted various managers and lawyers — proved unsuccessful, which isn’t surprising considering the irascible singer’s reputation for being media averse. In 2003, Morrison tried to block publication of Clinton Heylin’s biography “Van Morrison: Can You Feel the Silence?,” alleging that the book contained “outrageously false allegations” related to his taste for liquor and ladies.

“Writing this book was the hardest thing I’ve ever done, but also the most rewarding,” says Walsh.

The gratifying part was getting to play the Catacombs recording for Kielbania and Payne, who shared the stage with Morrison that night, calling themselves the Van Morrison Controversy. Though both were in New York a few months later when “Astral Weeks” was recorded, only Payne appears on the album. (Merenstein wanted ace jazz musicians, including bassist Richard Davis and guitarist Jay Berliner, to back Morrison in the studio.)

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A leaflet for the 1968 show during which Van Morrison debuted some songs from “Astral Weeks.”

“I didn’t even know who Van Morrison was,” says Payne, recalling the Catacombs show. “[Kielbania] invited me. I listened to the first set and didn’t particularly like it and almost left. But then they played the first song of the second set and I kind of got what Van was doing.”

Listening to the show 50 years later was surreal, he says.

“I was a little more daring than I thought I would’ve been,” says Payne, who runs a music school in Brookline. “Some of it I like and some of it is meh.”

Kielbania, who lives in Chicopee and has worked for a printer for 45 years, said it was strange to hear himself with Morrison after all this time.

“The acoustics aren’t exactly great on the recording, but as far as the music, that was the sound that spurred the whole ‘Astral Weeks’ thing,” he says.

Although Kielbania didn’t play on the record, he did tour with Morrison in 1969 to promote “Astral Weeks,” playing a show at the famed Whisky a Go Go in LA.

“We stayed across the street from where The Doors rehearsed, so I got to hang out and jam with The Doors,” says Kielbania. “That was fun.”

For his part, Walsh says he’s relieved to be finally done with the book, but admits he’s still curious about “Astral Weeks.” Surveying the books, notebooks, and binders of research materials that fill the shelves of his home office, Walsh sighs.

“Even after turning over every rock and learning everything I could,” he says, “the album still seems unknowable to me.”